WASHINGTON — The Justice Department on Wednesday called on Ferguson, Mo., to overhaul its criminal justice system, declaring that the city had engaged in so many constitutional violations that they could be corrected only by abandoning its entire approach to policing, retraining its employees and establishing new oversight.

In one example after another, the report described a city that used its police and courts as moneymaking ventures, a place where officers stopped and handcuffed people without probable cause, hurled racial slurs, used stun guns without provocation, and treated anyone as suspicious merely for questioning police tactics.

The report gave credence to many of the grievances aired last year by African-Americans in angry, sometimes violent protests after the deadly police shooting of Michael Brown, an unarmed black 18-year-old. Though the Justice Department separately concluded that the officer, Darren Wilson, who is white, violated no federal laws in that shooting, Attorney General Eric H. Holder Jr. said investigations revealed the root of the rage that brought people into the streets.

“Seen in this context — amid a highly toxic environment, defined by mistrust and resentment, stoked by years of bad feelings, and spurred by illegal and misguided practices — it is not difficult to imagine how a single tragic incident set off the city of Ferguson like a powder keg,” Mr. Holder said.

The findings will force Ferguson, a working-class city near St. Louis that is about two-thirds black but has a mostly white police force, to make changes or face a federal civil rights lawsuit. Justice Department officials, who met with city leaders to discuss their findings this week, said that it appeared that Ferguson was open to making changes that would head off a court battle.

In a statement he read to reporters Wednesday night, Mayor James Knowles said the city had begun making changes. He did not directly address all the steps that the Justice Department called for in the report or say whether he would fight the federal government in court.

“Today’s report allows the City of Ferguson to identify problems, not only in our Police Department but in the entire St. Louis region,” he said. “We must do better not only as a city, but as a state and as a country. We must all work to address issues of racial disparity in all aspects of society.”

Mr. Knowles did not answer questions from reporters.

It is rare for the Justice Department to bring the weight of the federal government down on a small city. Normally, it targets large police forces. But Mr. Brown’s shooting prompted a broader investigation, and Mr. Holder said what investigators uncovered raised questions about what went on in police departments around the country.

Their report described a city where police officers did not know the law or did not bother to follow it. Internal documents showed Ferguson police officers conducting “pedestrian checks,” in which they stopped people walking down the street and demanded to see their identification without any probable cause. One officer cited in the report told investigators he considered people who refused to show identification to be suspicious or aggressive, and typically arrested them.

When people refused to comply with — or even questioned — unconstitutional orders, police sometimes responded with force. Stun guns, for example, were commonly used even when officers were not threatened. “Supervisors seem to believe that any level of resistance justifies any level of force,” the report found.

Blacks in Ferguson accounted for 85 percent of traffic stops, 90 percent of tickets and 93 percent of arrests over a two-year period studied by investigators. In cases like jaywalking, which often hinge on police discretion, blacks accounted for 95 percent of those charged. A black motorist in Ferguson was twice as likely to be searched, according to the report, even though searches of whites turned up drugs and other contraband more often.

The Justice Department’s analysis found that these disparities could not be explained even when correcting for crime rates and demographics. “These disparities occur, at least in part, because Ferguson law enforcement practices are directly shaped and perpetuated by racial bias,” the Justice Department concluded.

Photo

Thomas Jackson, the Ferguson police chief, was surrounded by officers after a news conference in August. His department was accused of engaging in several questionable tactics.Credit
Robert Cohen/St. Louis Post-Dispatch, via Associated Press

In one example cited by the report, a police officer pulled up behind a 32-year-old black man who was cooling off in his car after a basketball game. Without cause, the officer demanded the man’s identification, ordered him out of his car, patted him down and asked to search the car. “The man objected, citing his constitutional rights,” the Justice Department wrote. “In response, the officer arrested the man, reportedly at gunpoint, charging him with eight violations of Ferguson’s municipal code.”

In July 2013, when officers arrived at an apartment building to arrest a suspect, they found an African-American man in the parking lot. The officers knew he was not the man they were looking for. “Nonetheless, without even reasonable suspicion, they handcuffed the man, placed him in the back of a patrol car, and ran his record,” the Justice Department found.

It turned out, he was the building’s landlord and he helped officers enter the building to make the arrest. The police department defended the detention as “minimal,” noting that the car was air-conditioned.

When officers do not have probable cause to make arrests, investigators found, they often use what are known as “wanteds.” By putting someone’s name in a police computer system, officers ensure he is arrested if he is ever stopped by the police in Ferguson or surrounding cities. “One veteran officer told us he will put out a wanted ‘if I do not have enough probable cause to arrest you,’ ” the Justice Department wrote.

After a local prosecutor wrote “we just don’t have enough for a warrant right now,” a detective responded that he would simply use a wanted to ensure the arrest.

Oversight in the department is so lax and record-keeping is so inconsistent that illegal stops and arrests go unnoticed or unquestioned. “Supervisors are more concerned with the number of citations and arrests officers produce than whether those citations and arrests are lawful or promote public safety,” the Justice Department concluded.

But federal authorities reserved some of their harshest criticism for the local court system, which does not function as an independent branch of government. Court employees answer to the police chief. The prosecutor is also the city’s lawyer. Judges are installed by the City Council and must be reappointed every two years.

The Justice Department report describes the courts as a bureaucratic morass in which people often receive the wrong court dates, court procedures are made up on the fly, and it can be unclear how much people owe or when they owe it. The punishment for missing a payment or an appearance, even for routine traffic violations, is often jail.

Court fines are a major source of revenue, and internal emails show city officials pushing for more tickets and fines, then congratulating one another when revenue exceeded expectations. Police supervisors insisted that officers hit ticket quotas and reorganized the shift schedules to help hit them.

Poor, mostly African-American residents have described being trapped in the court system for years as they are repeatedly jailed, even when trying to make payments. Meanwhile, the Justice Department found, police officers and city officials regularly fix tickets for each other and their friends.

Federal investigators also found that city officials circulated racist jokes on their government email accounts without fear of punishment. One portrayed President Obama as a chimpanzee. Another included a photograph of African women dancing topless along with the caption “Michelle Obama’s High School Reunion.” A third joked that a black woman should receive a crime-prevention award for having an abortion.

Senior city officials, including those who will be in charge of deciding whether to make changes in response to the Justice Department report, received the emails, federal officials said.

In his statement, Mr. Knowles said that one police official was fired for sending racist emails and two others were under investigation after city officials were told about the emails by the Justice Department.

“These actions taken by these individuals are in no way representative of the employees of the City of Ferguson,” he said.

“I think a lot of people need to lose their jobs,” said Alexis Templeton, a resident who has organized protests in Ferguson. “When you have people sending certain emails and saying certain things, and then they go out and police the community, which happens to be predominantly black, those stereotypes play into the work.”

Ms. Templeton said she thought Ferguson’s police department should be disbanded. Her distrust runs deep. Late last year, when her grandmother’s home in Ferguson was broken into, Ms. Templeton said she told her not to call the police, because “I don’t trust them to do their jobs at all.”The Justice Department called for an entirely new approach, one built upon community policing. It said Ferguson should change the way it stopped, searched and arrested people. That will require new work schedules and a focus on crime prevention and outreach. It will also require extensive retraining and, Justice Department officials said, some outside overseer to ensure Ferguson makes the changes.

For Mr. Holder, who declared last year that he stood with the people of Ferguson, the report culminates a tenure that has made civil rights investigations a priority. He is leaving office as soon as his successor is confirmed, probably within the coming weeks. His department has opened more than 20 investigations into police departments. But none has received the attention of Ferguson.

“With the conclusion of our investigations into these matters,” Mr. Holder said, “I again commit to the people of Ferguson that we will continue to stand with you and to work with you to ensure that the necessary reforms are implemented.”

On Wednesday night, there were two dozen protesters outside the police department, and officers made a handful of arrests of those standing in the streets.

Matt Apuzzo reported from Washington, and John Eligon from Ferguson, Mo.

A version of this article appears in print on March 5, 2015, on Page A1 of the New York edition with the headline: Ferguson Police Tainted by Bias, Justice Dept. Says. Order Reprints|Today's Paper|Subscribe